SAD (sometimes referred to as winter depression) is a condition in which people experience depression as a result of seasonal variations in light. Human brains have a circadian pacemaker that regulates many body functions. As the seasons change and light duration varies, the circadian pacemaker regulates seasonal behavior by transmitting a signal of day length to the pineal gland, which secretes the hormone melatonin. Melatonin secretion increases in the winter as the duration of light decreases. Evidence suggests that the melatonin signal of seasonal change is present in people with SAD but not in healthy volunteers; thus there is a possibility that seasonal changes which influence the duration of melatonin secretion control the course of illness in individuals with SAD. This study will determine whether propranolol can shorten the duration of melatonin secretion and mimic the effect of summer days to improve symptoms of depression in people with SAD.